The present invention relates to a fiber channel switch, which comprises a plurality of connection terminals respectively connected to a host computer and at least one physical storage unit via fiber channels and storage virtualization means for producing a virtual storage unit with a part or a whole of a storage area of the physical storage unit and making the host computer recognize the virtual storage unit, and a computer system using the fiber channel switch.
In the conventional computer system including a plurality of physical storage units, storage areas of the physical storage units are combined so as to make a host computer recognize as a virtual storage unit.
An example of such computer system employing the virtual storage unit is disclosed in Japanese Patent Gazette No. 2003-44421. The computer system is shown in FIG. 3. A plurality of host computers (node units) 1 and a plurality of storage units 2 are connected by a fiber channel switch (network switch) 3. A network processor 31 of the fiber channel switch 3 combines a part or a whole of a storage area of each storage unit 2 so as to constitute a virtual storage unit (virtual common disk) 5. The host computers 1 are capable of accessing to the virtual storage unit 5.
By the virtualization of the storage with the fiber channel switch, a user can optionally produce the virtual storage unit on the basis of uses of the host computer. By accessing to the virtual storage unit, the host computer can use the physical storage units without regard to storage capacities and connection types of the physical storage units.
Further, reliability of the storage virtualization can be improved by using a plurality of fiber channel switches. This structure is called multinode.
A conventional multinode computer system including a plurality of the fiber channel switches is shown in FIG. 4.
In FIG. 4, host computers 11a and 11b and a RAID storage unit 14, which includes physical storage units X and Y, are respectively connected to connection terminals 12a-12j of fiber channel switches 10a and 10b via fiber channels (FC).
The fiber channel switches 10a and 10b make the host computers 11a and 11b recognize virtual storage units A and B, which have the same constitution. Namely, the host computers 11a and 11b are capable of accessing to the virtual storage units A and B similarly via any fiber channel switches 10a and 10b. In the following description, the structure having the multiplex fiber channel switches for producing redundant routes is called “multinode”.
By employing the multinode structure, even if one of the fiber channel switches 10a and 10b breaks down, the computer system can be continuously operated. Therefore, failure resistance of the system can be improved.
In FIG. 4, each of the fiber channel switches 10a and 10b includes the virtual storage unit A, which is constituted by a part of a storage area a1 (an upper part in FIG. 4) of the physical storage unit X and a part of a storage area a2 (an upper part in FIG. 4) of the physical storage unit Y, and the virtual storage unit B, which is constituted by a part of a storage area b1 (a lower part in FIG. 4) of the physical storage unit Y and a part of a storage area b2 (a lower part in FIG. 4) of the physical storage unit X. The fiber channel switches 10a and 10b serve the virtual storage units A and B to the host computer 11a and 11b as accessible storage units.
The structures of the virtual units, which are produced by the fiber channel switches 10a and 10b, are not limited to the example shown in FIG. 4. A part or whole of storage areas of physical storage units may be optionally combined to produce virtual storage units.
In case of producing the virtual storage units in the multinode system, reserves of access rights (requests for occupying access rights) for accessing to the RAID storage unit (the physical storage units X and Y) must be adjusted between the fiber channel switches 10a and 10b. 
While one of the fiber channel switches is accessing to the RAID storage unit (the physical storage units X and Y), if the other fiber channel switch accesses to the RAID storage unit (the physical storage units X and Y), data accessed by one fiber channel switch will be damaged. Especially, in case of virtualizing the storage, the fiber channel switch rarely accesses to a plurality of physical storage units in answer to the access of the host computer to one of the virtual storage units. Therefore, the reserves must be simultaneously performed for the physical storage units.
Conventionally, the fiber channel switches 10a and 10b mutually communicate, by Ethernet (trademark) communication means 13a and 13b, so as to adjust the reserves (see FIG. 4).
For example, the fiber channel switch 10b notifies the reserve of the RAID storage unit (the physical storage units X and Y) to the fiber channel switch 10a before reserving the RAID storage unit. The fiber channel switch 10b waits for a response to the notice, then sends a signal for executing the reserve to the RAID storage unit (the physical storage units X and Y). If the fiber channel switch 10a reserves the RAID storage unit (the physical storage units X and Y) when the fiber channel switch 10a receives the notice, the fiber channel switch 10a waits until terminating the access, then responds to the notice. If the fiber channel switch 10a does not reserve, the fiber channel switch 10a responds at once.
By performing the communication between the fiber channel switches 10a and 10b via the Ethernet to adjust the reserves of access to the RAID storage unit (the physical storage units X and Y), damaging data can be prevented.
Note that, as shown in FIG. 4, the Ethernet means 13a and 13b are multiplexed. Even if one of the Ethernet means 13a and 13b breaks down, the communication of the reserve can be performed via the other Ethernet means. Namely, failure resistance of the Ethernet communication can be improved.
However, in the conventional computer system having the multinode fiber channel switches, when all of the Ethernet means 13a and 13b break down, the communication of the reserve states cannot be performed. Even if a plurality of Ethernet communication routes are provided, their hardware structures are similar, so they rarely simultaneously reach the ends of their usefulness. In case of performing the Ethernet communication via an intermediary device, e.g., hub, all of the Ethernet communication routes will be out of service due to failure of the intermediary device or a power source. To improve reliability, a plurality of intermediary devices and power sources may be employed, but a production cost must be increased.
In the computer system having the conventional multinode fiber channel switches, when the Ethernet communication is interrupted, the reserves cannot be adjusted, so the fiber channel switches independently access to the physical storage units.
While one of the fiber channel switches is accessing to one of the physical storage units, if the other fiber channel switch accesses to the physical storage unit, data accessed by one fiber channel switch will be damaged.